The invention relates to a method for closing a valved sack and to a closed valved sack.
Valved sacks are generally known and are formed of one or more layers of flexible material such as paper or plastic. The valved sack is provided with a filling opening, which is formed by opposite layers of material which are not joined when manufacturing the sack. Valved sacks are filled with bulk cargo, for instance building material like cement, or foodstuffs like flour, which is brought into the valved sack through the filling opening. The filling opening is shaped such that the opposite layers of material are pressed onto one another after filling the valved sack, in order to prevent the contents of the valved sack from streaming out. The filing opening thus acts as a one-way valve. In practice, however, it turned out that for many kinds of bulk cargo streaming out is not prevented to a sufficient extent. In those cases the valved sacks are closed by bringing the opposite layers of material against each other and joining them undetachably. With plastic valved sacks the connection is usually effected by thermo-welding or by applying glue, for instance a hotmeltglue. With paper valved sacks the opposite layers of material are usually glued to each other, or welded together by heat when they are provided with a (plastic) coating. Thus the filling opening cannot be opened and closed again after closing.
A disadvantage of these known methods is that in the case of welding heat is supplied, which may be detrimental to the quality of the contents of the valved sack, for instance foodstuffs. In the case of gluing it is the glue itself which may be detrimental to the quality of the contents. In both cases it is so that while filling remnants of the contents may be left behind in the filling opening, because of which the attachment of the heat weld or the glue can be insufficient for a good closing of the valved sack. The supply of glue or a (plastic) coating to a paper valved sack also has the disadvantage that the valved sack cannot be entirely recycled just like that.
It is an object of the invention to provide an improved closing of valved sacks.
To that end according to a first aspect of the invention a method is provided for closing a valved sack of one or more layers of flexible material such as paper or plastic, in which the valved sack has a filling opening which is closed after filling the valved sack, in which the filling opening is formed by opposite layers of material, which when closing the valved sack are on top of one another and are undetachably joined, characterized in that the filling opening is closed by joining the layers of material which are on top of one another by plastic deformation.
By mechanically joining the layers of material which are on top of one another, neither heat nor material alien to paper such as glue or a (plastic) coating has to be supplied. The mechanical connection can be quickly effected. By the plastic deformation remnants of the contents are either pushed away or included in the deformed layers of material, without notably influencing the closing function of the connection. Because hardly any forces which would open the filling opening act on a closed filling opening, plastic deformation of the layers of material offers a sufficiently firm connection for the desired closing of the valved sack.
Preferably the filling opening is closed by joining the material layers which are on top of one another with a connection in the form of a band along substantially the entire width of the filling opening. In this way the valved sack is closed entirely.
The connection in the form of a band is advantageously obtained by corrugating the layers of material on the spot, in which the corrugations provide a firm plastic deformation, which can be applied easily.
Preferably the plastic deformation is obtained by pressure welding or knurling the layers of material which are on top of one another. When pressure welding or knurling (compare to the German xe2x80x9cprxc3xa4genxe2x80x9d) a relief is pressed into the material, resulting in a mechanical connection between the layers of material which are on top of one another.
Preferably the valved sack is provided with a tube of flexible material such as paper or plastic arranged in the filling opening, which tube partially protrudes after filling the valved sack, in which after filling the valved sack the sides of the protruding portion of the tube that are on top of one another are joined by plastic deformation. After filling the protruding portion of the tube can be easily engaged by a machine.
More preferably the valved sack is a valved hexagonal bottom sack, of which the protruding portion of the tube is closed by plastic deformation, after filling the valved sack.
According to a second aspect of the invention a valved sack is provided of one or more layers of flexible material such as paper or plastic, provided with a filling opening formed by opposite layers of material, which valved sack is closed after filling, in which the opposite layers of material are on top of one another and are undetachably joined, characterized in that the layers of material which are on top of one another are joined by plastic deformation.
In this way a valved sack is provided, which without the supply of heat or adding material alien to paper provides a good closing against the streaming out of the contents of the valved sack. With paper sacks the valved sack remains entirely recyclable.
Preferably the plastic deformation extends in the form of a band along substantially the entire width of the filling opening, in which preferably a corrugated profile is applied and preferably the plastic deformation is effected by pressure welding or knurling.
According to a advantageous embodiment the valved sack is provided with a tube of flexible material such as paper or plastic, arranged in the filling opening, which tube partially protrudes after filling the valved sack, in which the tube is closed by plastic deformation of its sides which are on top of one another, and preferably the valved sack is a valved hexagonal bottom sack.